The general goal of this proposal is to demonstrate that fluorometric methods may be used to monitor membrane potentials in a eucaryote cell, the Ehrlich ascites tumor cell. Studies of the membrane potential will be made in conjunction with measurements of passive ion fluxes, co-transport and active ion transport in order to develop a quantitative expression for the potential difference across the membrane of this cell. These measurements and similar studies with membrane bound cytoplasts of the cells will be used to test the hypothesis that the energy for Na-dependent amino acid transport is derived solely from the electrochemical gradient of Na ion (the Na ion grandient hypothesis). The cytoplast preparation will be developed to avoid the complications of assessing the Na ion gradient if the nucleus sequesters Na ion. The influence of the membrane potential will also be taken into consideration in other aspects of Na ion-dependent amino acid transport including stoichiometry and the possible involvement of K ion.